Optical fiber cable and methods of making

ABSTRACT

An optical fiber cable (20) includes a core (21) comprising a units a unit (22). The unit is formed by a plurality of optical fibers (24-24) which are assembled together without intended standing. The unit is enclosed in a tube (34) which provides a predetermined packing density and which is substantially parallel to the longitudinal axis of the cable. In one embodiment, a waterblocking material (36) is disposed within the tube to fill the interstices between the optical fibers and between the unit and the tube. The waterblocking material is such that its critical yield does not exceed about 70 Pa at 20° C. and such that it has a shear modulus of less than about 13 KPa at 20° C. The tube is enclosed with non-metallic or metallic strength members and a plastic inner jacket and by another layer of strength members and by a plastic outer jacket. This arrangement is such that the cable may be manufactured and installed with the optical fibers capable of moving within the cable structure to prevent undue stresses being imparted thereto which would cause unwanted microbending losses.

This is a continuation of application Ser. No. 07/180,652 (U.S. Pat. No.4,826,278) filed Mar. 25, 1988 which is a continuation of applicationSer. No. 06/721,533 filed on Apr. 10, 1985 (abandoned).

TECHNICAL FIELD

This invention relates to an optical fiber cable and to methods ofmaking it. More particularly, this invention relates to an optical fibercable which includes a plurality of multifiber units which are disposedwithin a single tube that is provided with additional elements of asheath arrangement.

BACKGROUND OF THE INVENTION

Developments in the optical fiber communications field have been rapid.However, the technology still is undergoing major shifts in direction.For example, earlier generation fiber systems were designed to operateat wavelengths of about 0.8 μm, and current systems operate at 1.3 μm.Now there is growing interest in systems having an operating wavelengthof about 1.55 μm to take advantage of the loss window that exists insilica-based optical fiber in that wavelength region. Another example ofa major shift which is driven by demand for higher bandwidths is thatfrom multimode to single mode fibers.

Although desired for their large bandwidth capabilities and small size,light-transmitting optical fibers are mechanically fragile, exhibitinglow-strain fracture under tensile loading and degraded lighttransmission when bent. The degradation in transmission which resultsfrom bending is known as microbending loss. As a result, cablestructures have been developed to protect mechanically the opticalfibers.

A cable for use in a duct must be capable of withstanding tensile loadsapplied when the cable is pulled into the duct and stresses caused bybends. Cable structures which have been developed for optical fibersinclude loose tube, stranded and ribbon cables. For a description ofloose tube cables, see, for example, D. Lawrence and P. Bark "RecentDevelopments in Mini-Unit Cable" published at pp. 301-307 of theProceedings of the 32nd International Wire and Cable Symposium, 1983.See also U.S. Pat. No. 4,153,332.

Ribbon cable comprises one or more ribbons with each including aplurality of optical fibers disposed generally in a planar array. InU.S. Pat. No. 4,078,853 which issued to R. Kempf et al on Mar. 14, 1978is shown a cable which includes a core of ribbons surrounded by aloose-fitting plastic inner tubular jacket. A plastic outer jacket isreinforced with strength members which are encapsulated in the outerjacket to achieve tight coupling therewith.

In some situations, especially duct systems which include many bendssuch as those in loop plant in urban areas, greater tensile loads areexpected. An improved optical communications cable which is suitable forsuch use is disclosed in U.S. Pat. No. 4,241,979 which issued on Dec.30, 1980 in the names of P. F. Gagen and M. R. Santana. A bedding layer,about which strength members are wrapped helically, is added betweenplastic extruded inner and outer jackets to control the extent to whichthe strength members are encapsulated by the outer jacket. The cableincludes two separate layers of strength members, which are wrappedhelically in opposite directions. Under a sustained tensile load, thesetwo layers of strength members produce equal but oppositely directedtorques about the cable to insure the absence of twisting.

Ribbon cable has a number of attractive features. One is the relativeease of array connectorization. Array connectors shown, for example, inU.S. Pat. No. 3,864,018 can be factory installed and can save much timeover that required for single fiber joining techniques. A furtheradvantage is that a higher fiber density can be achieved per unit ofcable cross-section than in a stranded cable.

In another type of optical communications cable, a plurality of opticalfibers are enclosed in an extruded plastic tube to form a unit and aplurality of these tubed units are enclosed in a common extruded plastictube which is enclosed in a sheath system. Generally, the optical fiberswhich are enclosed in each unit tube are stranded together about acentral strength member. A central strength member is used because it isrelatively easy to assemble into the cable. Also, the cable is moreeasily bent if it has a central strength member rather than strengthmembers which are incorporated into the sheath system. However, whensuch a cable is bent, the central strength member may in some instancescompress one or more of the fibers against the tube and cause damagethereto.

Generally, optical fiber cables of the prior art, such as ribbon andstranded and loose tube, suffer from the disadvantage of having theribbons, the stranded units or the tubes manufactured on a separateline. In stranded cable, for example, a plurality of units which priorlyhave been enclosed individually in tubes and stranded are fed into aline which applies the common tube and the outer jacket. Each of theunits must be made separately on another line and inventoried until aplurality of them can be associated together in the common tube. Becausethe ribbon or tubed core is generally stranded with a predetermined lay,its manufacture and the assembly of the ribbons or tubes into the coreinvolves the use of relatively heavy rotating apparatus which isundesirable from a manufacturing standpoint.

Further complicating of the cable situation is the introduction of awaterblocking filling material into the cable core in order to preventthe incursion of water. A viscoelastic waterblocking material which hasbeen used in the past is disclosed in U.S. Pat. No. 4,176,240 issued onNov. 27, 1979, in the name of R. Sabia. Typically, the waterblockingmaterials in use do not yield under strains experienced when the cableis made or handled. This prevents the movement of the optical fiberswithin the cable and the fibers buckle because they contact, with arelative small periodicity, a surface of the unyielding fillingmaterial. The smaller the periodicity of the fibers in contacting suchan unyielding surface, the greater the microbending loss. This isovercome somewhat by stranding the cable which allows the fibers understress to form new helices to avoid microbending losses. A grease-likefilling composition having a relatively low critical yield stress isdisclosed in application Ser. No. 697,054 filed on Jan. 31, 1985, in thenames of C. H. Gartside III et al (U.S. Pat. No. 4,701,016).

Clearly, what is needed is a cable for optical fiber transmission whichdeparts from those used in the past. That cable should be one which canbe made inexpensively relative to present costs and which is relativelycompact. Also, the cable structure should be one which inhibits theintroduction of undue stresses which would lead to microbending lossesin the optical fibers. It is believed that the prior art does notinclude such a cable for which there is a long felt need in order toprovide low cost optical fiber communications.

SUMMARY OF THE INVENTION

The foregoing problems have been overcome by a cable of this invention.An optical fiber cable of this invention includes a plurality of opticalfibers which are assembled together without intended stranding to form aunit which extends in a direction along a longitudinal axis of thecable. A length of tubing which is made of a plastic material enclosesthe plurality of optical fibers and is parallel to the longitudinal axisof the cable. The ratio of the cross-sectional area of the plurality ofoptical fibers to the cross-sectional area within the tube does notexceed a predetermined value which in a preferred embodiment in whichthe optical fibers are coated is about 0.5. The cable also includes atleast one strength member and a jacket which is made of a plasticmaterial and which encloses the length of tubing. A length of tubingwhich is made of a plastic material encloses the plurality of units withthe length of the tubing being no greater than the length of the fibersin the unit. A waterblocking material which is disposed within thetubing and which fills substantially the interstices between the fibershas a critical yield stress which is not greater than about 70 Pa at 20°C. and a shear modulus which is less than about 13 Kpa at about 20° C.Each unit is separated from another unit only by the waterblockingmaterial and the plurality of units are enclosed in a common length oftubing instead of individual tubes as in the prior art. Thewaterblocking material is such that it behaves as an elastic solid up toa critical stress value, and is characterized as a liquid above thatvalue.

In a method of making an optical cable, a plurality of fibers are fedinto juxtaposition with one another to form a unit after which the unitis bound. A tube which is made of a plastic material is caused to beextruded about the unit, after which the cable is provided with astrength member. Afterwards, the tube is covered with a jacket which ismade of a plastic material.

In another embodiment, a plurality of the bound units are made on acommon manufacturing line and fed into an extruder which causes a tubemade of plastic material to enclose the plurality of units. As the tubeis being formed, a waterblocking material which has a critical yieldstress not greater than about 70 Pa at 20° C. and a shear modulus lessthan about 13 Kpa at 20° C. is introduced into the core. It fills theinterstices between the optical fibers and the units. Afterwards, asheath arrangement is caused to be disposed about the common plastictube, the sheath arrangement including at least one jacket and strengthmembers which are disposed between the tube and the outer surface of thejacket.

BRIEF DESCRIPTION OF THE DRAWINGS

Other features of the present invention will be more readily understoodfrom the following detailed description of specific embodiments thereofwhen read in conjunction with the accompanying drawings, in which:

FIG. 1 is a perspective view of an optical fiber cable;

FIG. 2 is an end view of the optical fiber cable of FIG. 1;

FIG. 3 is an end view of a coated optical fiber;

FIG. 4 shows an exemplary curve of applied stress versus strain for awaterblocking material for the cable of FIG. 1;

FIG. 5 is a schematic view of a manufacturing line which is used tomanufacture the cable of FIG. 1;

FIG. 6 is a perspective view of a portion of the line of FIG. 5 to showoptical fiber supplies and the assembly of pluralities of optical fibersinto separate units;

FIG. 7 is an end view of a portion of the apparatus of FIG. 5 to show afacility which is effective to adjust the core-to-sheath length ratio ofthe cable of FIG. 1;

FIG. 8 is a perspective view of a cable which includes one unit; and

FIG. 9 is a perspective view of still another embodiment of a cable.

DETAILED DESCRIPTION

Referring now to FIGS. 1 and 2, there is shown a cable 20. It includes acore 21 comprising at least one unit each of which is designatedgenerally by the numeral 22 and includes a plurality of individualoptical fibers 24--24. Further, each of the optical fibers 24--24includes a fiber 26 and one or more coatings 28--28 (see FIG. 3). Itshould be understood that herein the term optical fiber refers to thefiber itself and any coating applied thereto. Each of the units 22--22either is stranded or non-stranded, that is the unit extends generallyparallel to a longitudinal axis 29 of the cable, or is formed with anoscillating lay.

It should be understood that the optical fibers 24--24 which areincluded in each of the units 22--22 of the preferred embodiment areassembled without being stranded together and furthermore that the unititself is assembled with an infinite lay length. The optical fibers maybe undulated along portions of the unit which will cause each of theoptical fibers to have a length which is at least slightly greater thanthe length of any enclosing sheath system. This will prevent unduestrain from being imparted to the optical fibers 24--24 duringmanufacture, installation and service of the cable.

As is further seen in FIGS. 1 and 2, each unit is individually bound bya binder 32 and is enclosed in a tube 34. The tube 34 which is made of aplastic material such as polyvinyl chloride or polyethylene, forexample, contains the individually untubed, bound units and extendsgenerally parallel to the longitudinal axis 29 of the cable 20.

An important characteristic of the cable of this invention is itspacking density. Packing density is defined as the ratio between thecross-sectional area of the optical fibers and any coatings thereon tothe total cross-sectional area enclosed by the tube 34. If the packingdensity is too high, optical fibers within the core may experiencerelatively high stress and could break during handling of the cable.This occurs when the packing density is too high, because as with theuse of waterblocking materials which yield at a relatively high stress,the optical fiber cannot move sufficiently within the tube to relievestresses such as would occur in bending. In the prior art, this problemwas overcome by stranding the individual tubes which enclose the units.However, as is well-known, stranding requires a lower line speed andresults in increased costs.

In the embodiment shown in FIGS. 1 and 2, the at least one unit 22 andthe core between the at least one unit and the tube 34 are filled with asuitable waterblocking material 36. It has been determined that in anoptical fiber cable, a filling composition must also function tomaintain the optical fibers in a relatively low state of stress. Such amaterial is a colloidal particle-filled grease composition disclosed inpriorly disclosed application Ser. No. 697,054 which was filed Jan. 31,1985, (U.S. Pat. No. 4,701,016) and which is incorporated by referencehereinto.

A grease typically is a solid or semiliquid substance comprising athickening or gelling agent in a liquid carrier. The gelling agents usedin greases frequently are fatty acid soaps, but high melting pointmaterials, such as clays, silicas, organic dyes, aromatic amides, andurea derivatives also may be used.

When a low stress is applied to a grease, the material actssubstantially as a solid-like material. If the stress is above acritical value, then the viscosity decreases rapidly and the materialflows. The decrease in viscosity is largely reversible because typicallyit is caused by the rupture of network junctions between fillerparticles, and these junctions can reform following the removal of thesupercritical stress.

A cable filling or waterproofing material, especially an optical fibercable filling compound, should meet a variety of requirements. Amongthem is the requirement that the physical properties of the cable remainwithin acceptable limits over a rather wide temperature range, e.g. fromabout -40 to about 76° C. It is also desirable that the filling materialbe relatively free of syneresis over the aforementioned temperaturerange. Syneresis is the separation of oil from the gel under appliedstress. Filling materials for use in optical fiber cables also shouldhave a relatively low shear modulus. According to the prior art, theshear modulus is a critical material parameter of optical fiber cablefilling materials because it is believed to be directly related to theamount of microbending loss. For a discussion of microbending loss, seeS. E. Miller et al., Optical Fiber Telecommunications, Academic Press,New York (1979), pp. 158-161. Typically, microbending loss is moredifficult to control at long wavelengths than at short ones. Thus, it isimportant to be able to produce optical fiber cable that has nosignificant cabling-induced losses at long wavelengths such as, forexample, 1.55 μm.

The preferred waterblocking material is a composition which comprisestwo major constituents, namely oil, and a gelling agent such ascolloidal particles, and, optionally, a bleed inhibitor as a third majorcomponent. Other constituents such as a thermal oxidative stabilizer,for example, are optional.

Among the oils useful in the waterblocking material are polybutene oilshaving a minimum specific gravity of about 0.83 and a maximum pourpoint, as per ASTM D97, of less than about 18° C., or ASTM type 103,104A, or 104B, or mixtures thereof, per ASTM D-226 test, of naphthenicor paraffinic oils having a minimum specific gravity of about 0.86, anda maximum pour point, per ASTM D97, of less than about -4° C. Specificexamples of oils useful in the cable of the invention are a polybuteneoil, which is a synthetic hydrocarbon oil having a pour point per ASTMD97 of -35° C., an SUS viscosity of 1005 at 99° C., a specific gravityof 0.8509, and an average molecular weight of 460. It is available fromthe Amoco Chemical Corporation, Texas City, Tex., under the tradedesignation L-100. Another example oil is a white mineral oil, having apour point per ASTM D97 of -25° C., an SUS viscosity of 53.7 at 99° C.,an average specific gravity of 0.884, and maximum aromatic oils 1% byweight (b.w.). The latter is available from Penreco of Butler, Pa.,under the designation Drakeol 35. Other oils include triglyceride-basedvegetable oils such as castor oil and other synthetic hydrocarbon oilssuch as polypropylene oils. For applications requiring fire-retardantproperties, chlorinated paraffin oils having a chlorine content of about30-75% b.w. and a viscosity at 25° C. of between 100 and 10,000 cps areuseful. An example of such oil is Paroil 152, which is available fromthe Dover Chemical Company of Dover, Ohio. Polymerized esters of acrylicacid or similar materials are useful as pour-point depressants ataddition levels below 5% b.w. An example is ECA 7955, available from theExxon Chemical Company.

Colloidal filler particles in oil gel the oil by bonding surfacehydroxyl groups to form a network. Such gels are capable of supporting aload below a critical value of stress. Above this stress level, thenetwork is disrupted, and the material assumes a liquid-like characterand flows under stress. Such behavior is often referred to asthixotropic.

Colloidal fillers useful in the cable of the invention include colloidalsilica, either hydrophilic or hydrophobic, preferably a hydrophobicfumed silica having a BET surface area between about 50 and about 400 m²/gm. An example of a hydrophobic fumed silica is apolydimethylsiloxane-coated fumed silica having a BET surface area ofabout 80-120 m² /gm, containing about 5% b.w. carbon, and beingavailable from the Cabot Corporation of Tuscola, Ill. under the tradedesignation Cab-O-Sil N70-TS. An exemplary hydrophilic colloidalmaterial is fumed silica with a BET surface area of about 175-225 m²/gm, nominal particle size of 0.012 μm, and a specific gravity of 2.2,available from the Cabot Corporation under the designation Cab-O-SilM-5. Other colloidal fillers useful in the practice of the invention areprecipitated silicas and clays such as bentonites, with or withoutsurface treatment.

Oil-retention of the inventive greases may be improved by the additionof one or more bleed inhibitors to the composition. The bleed inhibitorcan be a rubber block copolymer, a relatively high viscosity semiliquid,sometimes referred to as semisolid, rubber, or other appropriate rubber.Block copolymers and semiliquid rubbers will be referred to collectivelyas rubber polymers. Incorporating a rubber polymer into the greasecomposition allows a reduction in the amount of colloidal particles thatmust be added to the mixture to prevent syneresis of the gel. Thisreduction can result in cost savings. Furthermore, it makes possible theformulation of nonbleeding compositions having a relatively low criticalyield stress.

Among the rubber block copolymers that can be used in waterblockingcompositions for the cable of the invention are styrene-rubber andstyrene-rubber-styrene block copolymers having a styrene/rubber ratiobetween approximately 0.1 and 0.8 and a molecular weight, as indicatedby viscosity in toluene at 25° C., of from about 100 cps in a 20% b.w.rubber solution. Exemplary block rubbers are (a) a styrene-ethylenepropylene block copolymer (SEP), unplasticized, having a styrene/rubberratio of about 0.59, a specific gravity of about 0.93, a break strengthper ASTM D-412 of 300 psi, and being available from the Shell ChemicalCompany of Houston, Tex., under the trade designation Kraton G1701; (b)styrene-ethylene butylene block copolymer (SEB), having a styrene/rubberratio about 0.41, and being available from the Shell Chemical Companyunder the designation TRW-7-1511; (c) styrene-ethylene butylene-styreneblock copolymer (SEBS), unplasticized, and having a styrene/rubber ratioof about 0.16, a specific gravity of about 0.90, 750% elongation, 300%modulus per ASTM D-412 of 350 psi, and being available from the ShellChemical Corporation under the trade designation Kraton G1657. Otherstyrene-rubber or styrene-rubber-styrene block copolymers arestyrene-isoprene rubber (SI) and styrene-isoprene-styrene (SIS) rubber,styrene-butadiene (SB) and styrene-butadiene-styrene (SBS) rubber. Anexample of SIS is Kraton D1107, and an example of SBS is Kraton D1102,both available from the Shell Chemical Company.

Among the semiliquid rubbers found useful in the practice of theinvention are high viscosity polyisobutylenes having a Flory molecularweight between about 20,000 and 70,000. Exemplary thereof is apolyisobutylene having a Flory molecular weight of about 42,600-46,100,a specific gravity of about 0.91, and a Brookfield viscosity at 350° F.(about 177° C.) of about 26,000-35,000 cps, and available from the ExxonChemical Company of Houston, Tex. under the trade designation VistanexLM-MS. Other rubbers which are considered to be useful are butyl rubber,ethylene-propylene rubber (EPR), ethylene-propylene dimer rubber (EPDM),and chlorinated butyl rubber having a Mooney viscosity ML 1+8 at 100° C.per ASTM D-1646 of between about 20 and 90. Examples of the above areButyl 077, Vistalon 404, Vistalon 3708, and Chlorobutyl 1066,respectively, all available from the Exxon Chemical Company. Also usefulare depolymerized rubbers having a viscosity of between about 40,000 and400,000 cps at 38° C. An example thereof is DPR 75 available fromHardman, Inc. of Belleville, N.J.

Oil-retention has been tested by using a procedure that substantiallycorresponds to the Rural Electrification Authority (REA) PE-89oil-retention test. If any measured amount of dripped oil is presentfrom this test, the grease composition is considered to have failed theoil-retention test. Another test comprises centrifuging a 30 gm sampleof a composition for 60 minutes at 10,000 rpm, and decanting andweighing any separated oil at the end of the test period. It has beendetermined that in order for the composition to have desirable oilretention up to about 60° C. or about 80° C., it should exhibit at roomtemperature oil separation not greater than about 7% and about 2.5%,respectively, as determined by the above centrifuge test.

The composition of the waterblocking material 36 is intended to blockeffectively entry of water into the core 21 while minimizing the addedloss to the cable in order to provide excellent optical performance.Although the oil retention characteristic of the composition is aconcern, the most important property is the optical performance of thecable 20.

Table I shows the effect of several different bleed inhibitors on oilseparation, for two different oils, Drakeol 35 and L-100. The threeblock copolymer-containing compositions comprise 92% b.w. oil, 6% b.w.Cab-O-Sil N70-TS colloidal filler, and 2% b.w. inhibitor. The semiliquidrubber-containing compositions LM-MS comprise 6% b.w. N70-TS colloidalfiller, the indicated amounts of the inhibitor, and 89 and 84% b.w. ofDrakeol 35.

                  TABLE I                                                         ______________________________________                                        Oil Separation                                                                              Drakeol-35 L-100                                                Inhibitor     % Separation                                                                             % Separation                                         ______________________________________                                        2%     SEP        2.5        0.7                                              2%     SEB        11         3.5                                              2%     SEBS       5          2                                                5%     LM-MS      7          --                                               10%    LM-MS      2          --                                               ______________________________________                                    

Table II shows data on oil separation for several compositions that donot include bleed inhibitors. It should be evident that the addition ofa bleed inhibitor is more effective than increasing the colloidalparticle content of the composition in preventing oil separation ordrip. Also, increasing the colloidal particle-content of a grease to thepoint where syneresis is avoided results in increased critical yieldstress. Thus to avoid syneresis altogether, the low values of criticalyield stresses needed in some instances may be unobtainable without useof bleed inhibitors. The data of Table II was obtained with N70-TScolloidal filler and Drakeol 35 oil.

                  TABLE II                                                        ______________________________________                                        Oil Separation                                                                ______________________________________                                        fumed silica (% b.w.)                                                                            6     7         8  10                                      oil separation (% b.w.)                                                                         36    28        20  14                                      ______________________________________                                    

FIG. 4 shows a generalized stress-strain curve 37 at constant strainrate for a thixotropic material such as that used as the waterblockingmaterial 36, and identifies several important parameters. In segment 38of the stress-strain curve 37, the material acts essentially as anelastic solid. The segment extends from zero stress to the criticalyield stress σ_(c). The strain corresponding to σ_(c) is identified asα_(c), the critical shear strain. By definition, the coordinates σ_(c),α_(c) indicate the onset of yielding and the quantity σ_(c) /α_(c) (ordσ/dγ for σ<σ_(c)) is known as the shear modulus (G_(e)) of thematerial.

The prior art teaches that filling materials for optical fiber cableneed to have low values of G_(e). However, it has been determined that,at least for some applications, a low value of G_(e) of the fillingmaterial is not sufficient to assure low cabling loss, and that afurther parameter, the critical yield stress, σ_(c), also needs to becontrolled. Typically, the critical yield stress of material accordingto the invention is not greater than about 70 Pa, measured at 20° C.whereas the shear modulus is less than about 13 kPa at 20° C.

A segment 39 of the stress-strain curve of FIG. 4 represents increasingvalues of incremental strain for increasing stress. The stress σ_(y) isthe maximum value of stress sustainable by the material at a givenstrain rate with α_(y) being the corresponding strain. For strains inexcess of α_(y), the stress at first decreases as shown by segment 40,becoming substantially independent of strain for still greater values ofstrain as shown by the segment 41. The waterblocking material thusexhibits a liquid like behavior for α>α_(y).

A filling composition for a filled cable 20 typically comprises betweenabout 77 and about 95% b.w. oil. If a bleed inhibitor is present and theinhibitor is a rubber block copolymer, then the oil content typically isbetween about 90 and about 95% b.w. On the other hand, if the bleedinhibitor is a semiliquid rubber, then the oil content typically isbetween about 77 and about 91% b.w. The composition further comprises atmost 15% b.w., preferably at most 10% b.w., of colloidal particles. Ifthe colloidal particles are fumed silica, then a typical range is from 2to about 10% b.w., with 5-8% b.w. being currently preferred for someapplications. The bleed inhibitor content of the composition istypically between about 0.5 and 15%, with the currently preferred rangefor block copolymer rubbers being between about 0.5 and about 5% b.w.,and for semiliquid rubbers being between about 3 and about 15% b.w.Optionally, the composition may also comprise minor amounts of anoxidative stabilizer and other additives. An exemplary stabilizer istetrakis methane, available from Ciba-Geigy under the trade designationIrganox 1010. Typically the oil, colloidal particles, and, if used, ableed inhibitor, account for about 99% b.w. or more of the totalcomposition.

Exemplary compositions that were studied are shown in Table III in partsby weight. The compositions were prepared by known methods, typicallycomprising blending oil, bleed inhibitor, antioxidant, and colloidalparticle material first at ambient temperature and pressure, then atambient temperature under a partial vacuum (typically less than about300 Torr). Some compositions, e.g. E, were heated to about 150° C. whilebeing stirred, and maintained at that temperature for about 4 hours. Theresulting compositions were evaluated, including a determination ofσ_(c) and G_(e) of some by cone-and-plate rheometry. An exemplarysummary of the properties also is presented in Table III with allmeasurements of σ_(c) and G_(e) being at 20° C.

Of the example compositions disclosed in Table III, example A ispreferred. The stress values designated (a) were determined withoutaging while those designated (b) were aged for the time indicated.Notwithstanding the use of bleed inhibitors in many of the examples ofTable III, some do not pass the drip test. However, cables filled withany of the compositions of Table III meet the requirements for opticalperformance.

The mechanical properties of the inventive composition are a function ofthe colloidal particle content. For example, it has been determined thatσ_(c) as well as G_(e) decreases with decreasing particulate content.

                                      TABLE III                                   __________________________________________________________________________    Compositions (parts b.w.)                                                     Examples                                                                              A  B  C  D  E  F  G  H  I  J  K  L  M                                 __________________________________________________________________________    Constituents                                                                  Oil     93 97.5                                                                             92.5                                                                             92 92 95.5     92 92 88 83 91.5                              (Drakeol 35)                                                                  Oil                       93 92                                               (L-100)                                                                       Coiloidal Filler                                                                      7.0   6.0                                                                              6.5                                                                              6.0   7.0                                                                              6.0                                                                              6.0                                                                              6.0                                                                              7.0                                                                              7.0                                                                              7.0                               (N70-TS)                                                                      Colloidal Filler                                                                         2.5         2.5                                                    (M5)                                                                          Bleed Inhibitor                                                                             1.5                                                                              1.5                                                                              2.0                                                                              2.0   2.0            1.5                               (Kraton G1701)                                                                Bleed Inhibitor                 2.0                                           (Kraton G1657)                                                                Bleed Inhibitor                    2.0                                        (TRW-7-1511)                                                                  Bleed Inhibitor                       5.0                                                                              10                                   (LM-MS)                                                                       Stabilizer                                                                            0.2                                                                              0.2                                                                              0.2                                                                              0.2                                                                              0.2                                                                              0.2                                                                              0.2                                                                              0.2                                                                              0.2                                                                              0.2                                                                              0.2                                                                              0.2                                                                              0.2                               (Irganox 1010)                                                                (a)σ.sub.c (Pa)                                                                 10 9.4                                                                              7.2                                                                              8.1                                                                              6.6   3.1            3.6                                                                              15                                (a)G.sub.e (Kpa)                                                                      1.8                                                                              .5 1.5                                                                              1.7                                                                              1.7   1.7            2.0                                                                              2.6                               time (hrs)                                                                            16    16 16 16    16             16 22                                (b)σ.sub.c (Pa)                                                                 10    13 14 15    17             6.9                                                                              27                                (b)G.sub.e (KPa)                                                                      1.8   1.8                                                                              2.0                                                                              1.8   2.2            1.8                                                                              3.0                               __________________________________________________________________________

Advantageously, the waterblocking material 36 which is used to fill thecore of of a cable of this invention yields at a low enough stress sothat the optical fibers 24--24 and units 22--22 are capable of movingwithin the core when the cable is loaded or bent. The yielding fillingmaterial allows the optical fibers to move within the tube 34 whichreduces the stress therein and lengthens the life of the optical fibers.

As mentioned hereinbefore, the cable of this invention may be made withthe units not being stranded together, as in the preferred embodiment,or stranded or with an oscillating lay. Of course, the non-stranded ispreferred inasmuch as the stranding apparatus may be eliminated and linespeeds increased.

The tube 34 may be considered as one element of a sheath system 42 ofthe cable 20. Returning now to FIGS. 1 and 2, it is seen that over thetube 34 are disposed other elements of a sheath system comprising abedding layer 43 and a group of reinforcing strength members 48--48, anintermediate jacket 50 of polyethylene, another bedding layer 52 andanother group of strength members 56--56 and an outer jacket 58. Bothjackets are made of polyethylene although other plastic materials may beused. Further, the materials for the jackets may differ. The strengthmembers are steel wires in the preferred embodiment. However, it isapparent that other materials, metallic and non-metallic, may be usedfor those members.

Referring now to FIG. 5 of the drawings, there is shown an apparatuswhich is designated generally by the numeral 70 and which may be used tomanufacture the cable of this invention. In the apparatus 70, a payout71 (see FIGS. 5 and 6) is provided for supplying a plurality of thecoated optical fibers 24--24 for each unit 22. Each of the fibers 24--24is payed out from a spool 73 which is mounted on a support platform 76.The supply spools 73--73 are mounted rotatably on either side of oralong an axis 79 of the manufacturing line and are braked to apply adesired back tension to the fibers.

Then the optical fibers 24--24 which comprise each unit are moved intogradual juxtaposition with one another and through a device 75 which iseffective to apply a binder 32 to the unit being assembled therein. Asmentioned hereinbefore, the optical fibers 24--24 are assembled togetherwithout an intentional stranding lay. In other words, the optical fibersare not stranded together and any lay generally is infinite. Of course,as the optical fibers 24--24 are assembled together, there may be somecrossover of the optical fibers to produce a modicum of unintendedstranding, but in which the relative positions of the optical fiberswill not change more than 360°.

After the optical fibers 24--24 have been assembled into units, they areadvanced through a guide tube (not shown) and a chamber, which resemblesa core tube and die cavity in a conventional crosshead extruder, inwhich the hereinbefore described waterblocking material 36 is introducedin and about the core. This causes the interstices among the opticalfibers 24--24 of each unit 22 of the core as well as the portion of thecable cross-section between the units 22--22 to be filled with thewaterblocking material 36. From the filling chamber, the filled core isadvanced through a core tube of an extruder 86 which is operative toextrude the plastic tube 34 about the units. This tube is common to allthe units 22--22.

As mentioned hereinbefore, it should be understood that the cable ofthis invention also could be what is referred to as an air core cablewhich does not include a filling material. In that cross-section, theunits 22--22, prior to their movement through the extruder 86, are movedpast a device 81 which supplies a tape 82 made of a plastic materialsuch as TEFLON® plastic. The tape is wrapped by well known methods aboutthe core to form a core wrap which functions as a thermal barrier toprotect the units during extrusion of the tube 34.

As the enclosed core 21 is advanced through the extruder 86, a plasticmaterial is caused to be extruded about the enclosed core to form theinner jacket 34 which is the common tube. From the extruder 86, thejacketed core is passed through a cooling trough 88 which is exposed toa negative pressure. This causes the tube 34 to expand away from thecore 21 and be sized externally as it cools. As a result, the tube 34 isformed about the core 21 with a packing density which permits relativemovement between the core and the tube and the subsequently appliedportions of the sheath system.

Afterwards, the cooled and jacketed core is moved through a device 91which applies the bedding layer 43 about the jacket. Then the jacketedcore is advanced through a payout assembly 95 for the longitudinalstrength members 48--48. The payout assembly 95 is disclosed in U.S.Pat. No. 4,446,686 which issued on May 8, 1984 in the names of A. J.Panuska, M. R. Santana and R. B. Sprow and which is incorporated byreference hereinto.

Mounted within the device 95 are a plurality of spools of lengths ofwires which form the strength members 48--48. A controlled amount ofback tension is applied to the spools. The payout assembly 95 iseffective to wrap the wires in spaced relation to each other helicallyabout the bedding layer 43.

The inner cable structure is moved through the payout assembly 95 andthen through the core tube of an extruder 101 which applies theintermediate jacket 50. At the same time, the strength members 48--48are fed from the supply spools and caused to be directed to travel insubstantially longitudinal paths adjacent to the inner cable structure.The strength members become disposed in the predetermined helical layprior to their entry into the extruder.

Following extrusion of the intermediate jacket 50 about the inner cablestructure and its cooling in a trough 103, the second bedding layer 52is applied by a device 105. Then the partially completed cable structureis advanced through a second strength member payout assembly 107 and anextruder 108 which are similar to the payout assembly 95 and theextruder 101. These are effective to cause the strength members 56--56to be wrapped helically about the intermediate jacket and the outerjacket 58 to be extruded thereover. Afterwards, the jacketed core isadvanced through a cooling trough 109.

Advantageously, cable 20 is a torque-balanced cable. The two layers ofstrength members are wrapped helically in opposite directions so thatunder a tensile load the two layers produce equal but oppositelydirected torques about the longitudinal axis of the cable. Thiseliminates twisting that can otherwise occur when the cable 20 isexposed to substained tensile loads.

After the extrusion of the outer jacket 58, but before the cable 20 istaken up, the core 21 is caused to be coupled to the sheath system 42after the sheath has been elongated by a predetermined amount. Couplingas applied to the manufacture of the cable 20 connotes that the relativemovement between the core 21 and the sheath system 42 is substantiallyzero. This is accomplished in accordance with the methods and apparatusdisclosed in U.S. Pat. No. 4,446,686 which has been incorporated byreference hereinto.

An apparatus 110 (see FIG. 5) is effective to elongate the sheath system42 by the application of tensile forces to allow the core 21 to moverelative to the sheath system. After the sheath system 42 has beenextended relative to the core 21, the core is coupled to the sheathsystem to prevent relative movement between the core and the sheathsystem. The coupling is temporary inasmuch as the core 21 issubsequently decoupled from the sheath system 42 so that relativemovement can occur therebetween. The sheath system 42 recoverselastically in the absence of tension because of the composite structurewhich includes the strength members having a relatively high modulus ofelasticity. The sheath extension and the coupling are accomplished sothat when the stretched sheath system 42 recovers, its resultant ratioof the lengths of the core 21 and the sheath system 42 is apredetermined value. The recovery of the sheath system occurs to asubstantial degree as the cable 20 is taken up, and by the time thecable is laid out and installed in the field, the core-to-sheath lengthis as desired.

The coupling apparatus 110 includes a linear capstan 112 of and couplingsheave 115 (see FIGS. 5 and 7) which cooperate to produce a couplingafter the sheath has been stretched and while it is under tension.Cooperating with the linear capstan 112 to effect the sheath elongationand coupling is the sheave 115. The sheave 115 is mounted rotatablybetween the side supports of a stand 117 and is power driven by a motor.From the sheave 115, the cable 20 is advanced to a takeup reel 120. Thelinear capstan 112 is a constant speed apparatus, but the rotationalvelocity of the coupling sheave 115 is variable. Through a feedbackcontrol system, the sheave velocity is controlled to obtain a desiredtension in the sheath system 42.

The sheave 115 includes guide means for causing each successiveconvolution of the cable 20 on the sheave to be moved transverselyacross a surface 122 of a hub 124 of the sheave. A fin 126 (see FIG. 7)is mounted in a frame 128 and is adjacent to the hub 124. As the sheave115 is turned rotatably, the fin 126 causes the convolutions of thecable 20 to be separated and each successive one to be moved toward oneof flanges 129--129 of the sheave. The movement of the convolutionsacross the hub 124 from a point on an exit point is helped by taperingthe hub 122 (see FIG. 7) so that its largest diameter is adjacent to theentry point of each convolution and smallest at the exit.

The core 21 is coupled to the sheath system 42 after the sheath systemhas been elongated between the linear capstan 112 and the large sheave115. The amount of the strain in the sheath system 42 between the linearcapstan 112 and the sheave 115 is equal to the total strain of the core21. That total strain is the sum of geometric strain, a strain caused byfiber payout tension, and, if desired, an increment which insures thatthe length of the core 21 in the final product exceeds the length of thesheath system 42 in order to obtain a core-to-sheath length ratiopreferably between 1.0000 and about 1.0015. The tension in the sheathsystem 42 can be controlled by controlling the rotational velocity ofthe sheave 115 to compensate for the geometric strain, which comes aboutbecause the fiber center is displaced from the center or neutral axis ofthe sheath, as well as the other strains. The required tension will varyinasmuch as the geometric strain varies as a function of the number ofunits 22--22 in the core 21.

The cable 20 is wound, with its sheath system 42 in a stretchedcondition, on the sheave 115 for a number of turns sufficient to achievea coupling of the sheath system 42 and the core 21 while the sheath isin the stretched condition. It has been found that three turns aresufficient and that five are more than adequate. In the apparatus 70,there is a back tension on the core 21. The coupling between the core 21and the sheath system 42 is sufficient so that the back tension on thecore cannot cause the core to slip relative to the sheath to which it iscoupled. A relatively small force is required on the output side of thecoupling arrangement to balance the back tension on the other side. Thissmall tension on the output side is provided in a relatively smalllength of the cable between the sheath 115 and the takeup reel 120 bythe weight and friction of the fiber core 21.

Important to the successful coupling of the core 21 to the sheath system42 is the requirement that the diameter of each of the convolutions ofthe cable 20 on the coupling sheave 115 be larger than the diameter ofany other convolution in which the cable is subsequently wound. It hasbeen found that successively increasing convolution diameters results inthe application of increased tensile forces to the core 21. Theincreased tensile forces applied to the core 21 in such an arrangementcauses the core to be held against the sheave side of the tube 34thereby increasing the geometric strain. The movement of the cable 20past sheaves which increase in diameter between at least two successivesheaves also results in an increasing shortfall of the core. Thiscondition is referred to as pumping and is cumulative. The couplingsheave 115 has a diameter larger than all succeeding sheaves and largerthan any subsequent convolution in which the cable 20 is wound, whetherit is coupled or decoupled.

It also is important that the diameter of the coupling sheave 115 berelatively large. As will be recalled, the core 21 tends toward theinner side of the surrounding tube 34 as the cable 20 is moved about asheave. The desired location is along the neutral axis of the cable 21.The larger the sheave, the more each increment of the cable 20approaches a straight line. For an infinitely large diameter sheave, thecore of each increment approaches a length equal to an increment alongthe neutral axis. Also, a relatively large diameter sheave 115 is usedbecause the smaller the sheave diameter, the greater the force requiredto compensate for the shortfall of the core 21. Shortfall is independentof the sheave diameter; however, when the shortfall is expressed as apercent of the sheave circumference to arrive at the geometric strain,the percent becomes dependent on the diameter of the sheave. Therefore,for a smaller diameter sheave, the elongation of the sheath system 42must be greater than that for a larger sheave. As a result of thegeometric strain increasing as the diameter of the sheave 115 decreases,the tensile force required to elongate the sheath 42 to compensate forthe geometric strain increases. The use of a coupling sheave which in apreferred embodiment has a diameter of about nine feet overcomes theseproblems.

The coupling is temporary. When the cable 20 is substantially free oftension, the core 21 and the sheath system 42 are decoupled because ofthe elastic recovery of the composite sheath system to its originallength, and the lengths of the two are substantially equal. In thepreferred embodiment, the cable 20 is decoupled as it is advanced offthe sheave 115 and moved to the takeup reel 120. The tension in thecable core 21 between the coupling sheave 115 and the takeup reel 120 isessentially zero. The sheath system 42 on the takeup reel typically isunder a relatively small tension which is sufficient to provide a takeuppackage that is suitable for shipping and/or subsequent handling. By thetime the cable 20 has been wound on the takeup reel 120, sufficient core21 has been carried forward with the sheath system 42 on the output sideof the sheath 115 to cause the ratio of core and sheath lenghts to be apredetermined value, and the core and the sheath system aresubstantially decoupled.

What is generally sought is a zero difference in length between thecable sheath system 42 and the core 21 when the cable 20 is installed inthe field. Removing substantially all the tensile forces causes thesheath system 42 to recover by an amount equal to the sum of the coregeometric and payout strains to cause the core and sheath lengths to besubstantially equal. However, as will be recalled, in the preferredembodiment, the sheath system 42 is elongated by an amount equal to thetotal core strain which includes a predetermined increment in additionto the geometric strain. As a result, when the sheath system 42 recoverselastically, the core length exceeds the sheath length slightly and thecore is under a slight compressive load.

The cable of this invention is a solution to problems encountered withprior art cables. Because it has a common tube which encloses the units,it is relatively compact. Because the fibers and, in a preferredembodiment, the units, are not stranded, the manufacturing process isless expensive than for other cables. Also, there is substantially noadded loss as a result of assembling the optical fibers into a cable. Ithas been found that waterblocking materials of the cable of thisinvention with σ≦70 Pa resulted in a cable having a mean added losswhich is less than about 0.1 dB/km. Further, the cable possessesexcellent mechanical characteristics. Inasmuch as there is no internalstrength member, the possibility of the strength member damaging thefibers has been eliminated.

As mentioned hereinbefore, the cable is not restricted to one that isfilled. For an air core cable 130 (see FIG. 8), a plastic core wrapmaterial 132 is introduced between a core 133 and a plastic tube 134.The cable 130 includes at least one unit 136 which includes a pluralityof optical fibers 24--24 which are secured together with a binder 138.As in the filled cable 20, the optical fibers 24--24 of the cable 130have a generally infinite lay length, or, in other words are notintentionally stranded together. Portions thereof may, however, becomedisposed in an undulated configuration. The remaining portions of thecable 130 are designated with the same numerals as that in FIG. 1.

Although the optical fibers 24--24 are not stranded together, the units136--136 may have an oscillating lay or be stranded together (see FIG.9). In that arrangement, the units are secured together with a binder.

CABLE EXAMPLE 1

In a cable two units each of which included twelve single mode,depressed cladding optical fibers having an outer diameter over thecoating of 0.0096 inch were assembled. Neither the units nor the opticalfibers were stranded. A plastic material comprising a compositin basedon a polyvinyl chloride (PVC)-ethylene vinyl acetate (EVA) graft polymerwas extruded about the units to form a tube having an inner diameter of0.170 inch while a waterblocking material of example A in Table III wasintroduced into the core area. The sheath system was of the steelreinforced cross-ply type shown in FIG. 1 and had an outer diameter of0.41 inch. The mean added microbending loss of this cable at 1310 nm and1550 nm was 0 dB/km. The final cable losses at 1310 nm and 1550 nm were0.38 dB/Km and 0.24 dB/km, respectively.

CABLE EXAMPLE 2

Another cable having the same structure as that in Example 1 wasprovided with an oversheath comprising a copper shield, a stainlesssteel laminate and another polyethylene jacket. The oversheath providesprotection against rodents and lightning. Again, the added loss both at1310 nm and 1550 nm was 0 dB/km. The final cable loss was determined tobe 0.38 dB/Km at 1310 nm and 0.22 dB/km at 1550 nm.

It is to be understood that the above-described arrangements are simplyillustrative of the invention. Other arrangements may be devised bythose skilled in the art which will embody the principles of theinvention and fall within the spirit and scope thereof.

What is claimed is:
 1. An optical fiber cable which includes alongitudinal centerline axis, said optical fiber cable comprising:a corewhich has a longitudinal centerline axis which extends colinearly withthe longitudinal centerline axis of the cable, said core comprising: aunit which comprises a plurality of optical fibers which are assembledtogether without intended stranding to form said unit which extends in adirection substantially along a longitudinal axis of the cable; a tubewhich is made of a plastic material and which encloses said unit withthe ratio of the cross-sectional area of the plurality of optical fibersto the cross-sectional area within the tube not exceeding apredetermined value, said tube extending colinearly with and beingsubstantially parallel to the longitudinal centerline axis of the cable;and p1 a waterblocking material which is disposed within said tube andwhich fills substantially the interstices between the optical fibers andbetween said unit and said tube; a strength member; and p1 a jacketwhich is made of a plastic material and which encloses said core.
 2. Thecable of claim 1, wherein said waterblocking material has a criticalyield stress and a shear modulus which are such as to allow movement ofsaid unit within said tube.
 3. The optical fiber cable of claim 2,wherein the waterblocking material has a critical yield stress which isnot greater that about 70 Pa at 20° C. and a shear modulus less thanabout 13 KPa at 20° C.
 4. The optical fiber cable of claim 3, whereineach of the optical fibers is provided with a coating, wherein thepredetermined value is 0.5 wherein the length of the tube is no greaterthan the length of fibers in each unit.
 5. The cable of claim 4, whereinthe waterblocking material is a composition of matter whichcomprises:(a) 77 to 95% by weight of an oil selected from the groupconsisting of:i. paraffinic oil having a minimum specific gravity ofabout 0.86 and a pour point less than -4° C. and being of ASTM type 103,104 A or 104B; ii. naphthenic oil having a minimum specific gravity ofabout 0.86 and a pour point less that -4° C. and being of ASTM type 103,104A or 104B; iii. polybutene oil having a miniumum specific gravity ofabout 0.83 and a pour point less than 18° C.; and iv. any mixturethereof; and (b) 2 to 15% by weight of hydrophobic fumed silicacolloidal particles.
 6. The cable of claim 4, wherein the waterblockingmaterial is a composition of matter comprising:(a) 77 to 95% by weightof an oil selected from the group consisting of:i. paraffinic oil havinga minimum specific gravity of about 0.86 and a pour point of less than-4° C. and being of ASTM type 103, 104 A or 104B; ii. naphthenic oilhaving a minimum specific gravity of about 0.86 and a pour point lessthat -4° C. and being of ASTM type 103, 1104A, or 104B; iii. polybuteneoil having a minimum specific gravity of about 0.83 and a pour point ofless than 18° C.; iv. triglyceride-based vegetable oil; v. polypropyleneoil; vi. chlorinated paraffin oil having a chlorine content betweenabout 30 and 75% by weight and a viscosity at 25° C. of between 100 and10,000 cps; vii. polymerized esters, and viii. any mixture thereof; and(b) 2 to 15% by weight colloidal particles selected from the group,consisting of hydrophobic fused silica, hydrophillic fused silica,precipitated silica, and clay, the colloidal particles having a BETsurface area in the range from about 50 to about 400 m² /g.
 7. The cableof claim 6, wherein the composition of matter further comprises up to15% by weight of a bleed inhibitor selected from the group consisting ofstyrene-rubber and styrene-rubber-styrene block copolymers having astyrene/rubber ratio between about 0.1 and 0.8, semiliquid rubber havinga Flory molecular weight between 20,000 and 70,000, butyl rubber,ethylene-propylene rubber, ethylene-propylene dimer rubber, chlorinatedbutyl rubber having a Mooney viscosity at 100° C. between about 20 and90, and depolymerized rubber having a viscosity at 38° C. between 40,000and 400,000 cps;with the oil, the colloidal particles, and the bleedinhibitor comprising at least 99% by weight of the composition ofmatter.
 8. The cable of claim 2, wherein said composition comprisesabout 90 to 95% b.w. of oil and about 2 to 10% b.w. of colloidalparticles.